1. Field of the Invention
This invention relates generally to optical and radio instruments, and more particularly, to a device usable for reducing background electromagnetic radiation interferometrically in optical or radio frequency instruments.
Whenever an object is observed through an optical device, or when a signal is detected by an electronic instrument, unwanted background electromagnetic radiation is generally present along with the image of the desired object or desired signal, and tends to obscure the image or signal, particularly if the image or signal is weak. Background radiation arises from sources both internal and external to the optical or electronic instrument. In optical instruments, the internal sources of unwanted background radiation include reflection between the optical elements, diffraction by the effective aperture stop of the optical instrument, lens irregularities, such as bubbles or other inclusions, dust and scratches on the lens surface, and thermal radiation in infrared work. The major external source of scattered light in an optical system is the atmosphere, dust and other particles suspended within the atmosphere, and thermal radiation in the infrared. In electronic instruments, background radiation, or noise, is generated within the amplifier and detector stages of the instrument itself, and radiation external to the instrument includes atmospheric radiation, and radiation from man-made sources, such as various electronic and electrical equipment.
While in many instances, the magnitude of the unwanted background electromagnetic radiation is low relative to the brightness of the optical image or the magnitude of the signal, and does not adversely affect the quality of the image or signal, in other instances, where the brightness of the desired image is quite low, the background radiation can obscure the object or the signal. Such a problem is particularly acute in long distance instruments, such as astronomic telescopes, both optical and radio telescopes, long range receivers, and instruments used near light and radio noise generating urban areas.
2. Description of the Prior Art
Several methods for reducing background radiation are known. These include the use of various occulting discs and coated optical surfaces to reduce surface reflectivity in optical instruments, and low noise super-cooled amplification stages in electronic instruments to reduce internally generated undesired radiation. Diversity systems wherein signals received by two separate receivers are coherently combined have been used to reduce the effects of external radiation in electronic systems, and a system used to reduce the effects of both internally and externally generated background radiation utilizing interferometric techniques is described in U.S. Pat. No. 4,213,706.
While these techniques substantially reduce the amount of background radiation, the implementation of such techniques requires the use of apparatus that is costly, cumbersome and requires a high degree of maintenance. More importantly, none of these techniques, with the exception of diversity systems and the technique disclosed in U.S. Pat. No. 4,213,706, are able to effect any reduction in externally generated background radiation. However, while the system described in U.S. Pat. No. 4,213,706 does reduce the effects of externally generated radiation, including on-axis and off-axis radiation, its ability to reduce the effects of radiation that is substantially off-axis is not optimal. Also, diversity systems are generally complex and costly, and do not provide an appreciable increase in signal to noise ratio.